You would like to understand the difference between "Send email" and "Delivered email" activity.
Send email activity confirms that the email which you have sent has been dispatched from Marketo to be delivered.
Delivered email activity is recorded when the recipient email server responds that the email has been accepted for delivery.
Delivered email activity doesn't actually mean that email has been delivered to lead's email inbox. For more information on what happens to the email once it reaches the recipient email server, check out this blog post: https://nation.marketo.com/community/support_solutions/blog/2016/02/26/between-the-delivery-and-the-...